I. Field of the Invention
This invention pertains to a passive optical network (PON) which provides for the transmission and reception of upstream and downstream data for communicating data between a host terminal complex (HTC) and a plurality of optical network units (ONUs). More particularly, the present invention relates to a bi-directional passive optical communication network for transmitting downstream dense wavelength division multiplexed (WDM) optical signals contained in a first frequency band to ONUs and for transmitting upstream optical data signals contained in a second frequency band to an HTC by using relatively inexpensive optical sources such as wideband (Fabry-Perot) lasers.
II. Description of the Related Art
Passive optical communication networks have gained increased importance for communicating data to and from remote locations. Such systems communicate information from central offices to individual living units (e.g. subscribers' homes), and from living units to the central offices. Current preferred PONs that transmit data from a host terminal complex over fiber optic cables to designated ONUs which service designated subscribers' terminal devices employ wavelength division multiplexing (WDM) techniques to transmit a plurality of optical data signals in the form of a plurality of wavelengths. WDM systems are preferred because, unlike time division multiplexing or power splitting systems, a designated wavelength is provided only to a corresponding ONU and not to the other ONUs connected to a common remote terminal. Thus, privacy is enhanced because each user location receives only its designated signal or channel. WDM systems are also preferred because each wavelength only carries information for a single ONU and, thus, the bit rate for each wavelength can be lower.
A prior art PON system for the downstream transmission of data is depicted in FIG. 1. As shown, PON 10 includes a multi-wavelength source (MS) such as a simultaneous multi-wavelength source (SMS) 12 contained in a host terminal complex (HTC) 13 which receives data, represented by incoming signals X.sub.1 -X.sub.n. The incoming data X is converted to optical signals, in a manner well known to those having ordinary skill in the art and as for example more fully described in M. Zirngibl, et al., "Demonstration Of A 9.times.200 Mbit/s Wavelength Division Multiplexed Transmitter," Electronics Letters, pp. 1484-1485, 1994. Each incoming data stream is represented by a separate and discrete wavelength .lambda., shown as .lambda..sub.1 -.lambda..sub.n. The wavelengths are carried on a connection fiber 16 which provides a conduit for the data wavelengths between the SMS 12 and a router 14.
Router 14 contains an upstream port 18 and a plurality of downstream ports 20.sub.1 -20.sub.n for routing downstream signals generated by the SMS 12 to select ones of a plurality of drop fiber lines 22.sub.1 -22.sub.n. The operation of a suitable router 14 is more fully described in U.S. Pat. No. 5,136,671. In general, router 14 contains multiple optical paths, each of which exhibits a particular passband. Each passband permits the passage of one or more particular wavelengths along the respective optical path, to the substantial exclusion of others. Thus, router 14 divides or separates the wavelengths present at the upstream port 18--which are contained in a common frequency band--into a plurality of discrete wavelengths and places each discrete wavelength on a designated output port 20. As shown, each drop fiber line 22 is connected to an optical network unit (ONU) 24 containing an optical receiver 26 for retrieving the data input to the SMS 12. Thus, the output of each ONU 24 shows the data signal X corresponding to the data signal input to SMS 12.
As will be appreciated, it is necessary for such PON systems to operate in a bi-directional manner. In other words, it is desirable to also communicate data in the upstream direction, i.e. for data generated from each ONU and received by the HTC 13. The upstream data is powered by a light source, such as an LED or laser. Although some prior art techniques exist for accomplishing this, the prior art techniques (which are discussed more fully below) all suffer various drawbacks which, inter alia, limit the capacity and rate of upstream data.
For example, bi-directional PON systems exist wherein a separate fiber network identical to PON 10 is constructed in the upstream direction except that a wavelength independent combiner is substituted for an upstream router. The wavelength independent combiner will pass substantially all wavelengths within a particular bandwidth. Such a system thus permits the use of less expensive lasers, i.e. lasers that need not be as finely tuned to the specific wavelengths which carry the upstream data, e.g. .lambda..sub.1 '-.lambda..sub.n ', to pass through a bi-directional router. The combiner will combine or multiplex the various wavelengths (.lambda.') and provide the combined signal to a receiver contained in the HTC for retrieval of the upstream transmitted data. A significant drawback of such a duplicative system lies in its cost since duplicate upstream drop line fibers 22 are required for carrying the upstream optical signals from each ONU to the combiner.
Another prior art technique for transmitting upstream data through a PON is to incorporate at each ONU a separate single frequency laser such as a distributed feedback (DFB) laser. The type of laser used is dictated by the properties of the prior art routers. For example, conventional prior art routers are transparent to light only within a specified bandwidth. In other words, the router will transmit more power at some wavelengths than at other wavelengths to the upstream port. Accordingly, this property dictates the use of single frequency lasers as opposed to wideband Fabry-Perot lasers because single frequency lasers emit light at discrete wavelengths and may be finely tuned. Thus, light at specific wavelengths suitable for the router can be generated. In contrast, Fabry-Perot lasers generate light at various distinct frequencies or longitudinal modes and the optical power jumps between the modes in an unpredictable manner. Thus, it is difficult to adjust a Fabry-Perot laser so that even a portion of the generated light will reliably go through a conventional prior art router. Each DFB laser incorporated in such a system will be tuned to the specific ONU's transmitting wavelength. Thus, ONU 24.sub.1 will incorporate a DFB laser for generating light at .lambda..sub.1 '; ONU 24.sub.n will incorporate a DFB laser tuned to .lambda..sub.n ', etc. While such a system does not require duplicate drop lines to transmit the upstream data from the ONU 24 to the router 14, the multiple DFB lasers that are required are quite costly, and thus, significantly add to the expense of the overall PON system. In addition, the single frequency lasers require a costly mechanism to insure they remain at the desired wavelength.
An additional prior art technique used for transmitting upstream information in a PON is to employ a reflective modulator at each ONU for reflecting and modulating the downstream optical signal or light in the upstream direction. The reflected light will, of course, be at the same wavelength as the downstream transmitted light and thus, will pass through the router for detection by a receiver in an HTC, whereupon the reflected light is demodulated to retrieve the transmitted upstream data. The main drawback associated with this technique, however, is that the reflected light must travel through the network twice, i.e. from the HTC to the designated ONU and then back to the HTC for reception. This makes such a system overly susceptible to fiber losses. In addition, these systems often require separate upstream and downstream fibers to avoid effects of reflection and Rayleigh scattering.
Still another technique in the prior art to transmit upstream data through a PON lies in the use of a broadband light source such as an LED contained in each ONU. The broadband source must be broad enough so that the router is transparent to some of the light, i.e. the router will pass one band of light going downstream and at least a portion of the light going upstream. The main drawback associated with such a system is that LEDs have low fiber coupled power. Thus, they do not generate enough power at the specific wavelengths carrying the upstream data. In addition, most of the broadband light is not passed by the router, and is thus wasted.